1. Field of the Invention
This invention concerns a thick film conductor composition for a ceramic wiring substrate which is used as a high-density circuit mounting board for IC, LSI, chip components and the likes.
2. Description of the Prior Art
Noble metals such as Au, Au-Pt, Ag-Pt and Ag-Pd, or high melting point base metals such as W, Mo and Mo-Mn have generally been used as metals for conductor pastes used in ceramic wiring substrates.
The paste made of noble metals such as Au, Au-Pt, Ag-Pt and Ag-Pd can be fired in air but they have a drawback of expensive cost. While on the other hand, the paste made of high melting point base metals such as W, Mo and Mo-Mn can be formed into a multilayer structure since they are co-fired at a temperature of about 1600.degree. C., that is, a temperature higher than the sintering temperature of a green sheet (about 1500.degree. C.), but they show lower electric conductivity as compared with Au, Ag and Cu, and are required to be sintered at a high temperature in a hydrogen atmosphere, which is dangerous and needs an increased installation cost. Further, these high melting point base metals also involve a problem of requiring surface coating of Ni or Au to the conductor since they cannot be soldered.
In view of the above, Cu-metallized wiring substrates have being attracting attention at present, which are inexpensive, low in conductor resistance, free from migration and satisfactory in solderability. An Example method of manufacturing the Cu-metallized wiring substrate will be described. In the conventional method, a Cu paste is used as the material for a conductor layer. The Cu paste is screen-printed on a sintered substrate made of such as alumina to form a wiring pattern, and then dried. Then, the paste is fired at a temperature lower than the melting point of Cu (1083.degree. C.) in a nitrogen atmosphere where the oxygen partial pressure is controlled in a delicate manner such that Cu does not suffer from oxidation while the organic ingredient in the conductor paste is burnt out adequately. In the case of preparing a multilayer structure, an insulation paste and the Cu paste are further used, for which printing, drying and firing in a neutral atmosphere are repeated desired times.
The Cu paste as described above is prepared by adding an organic ingredient composed of an organic binder, a plasticizer, a solvent, etc. to the inorganic ingredient mainly composed of fine Cu powder and mixing these into an appropriate viscosity. U.S. Pat. No. 2,993,815 (Treptow) discloses a Cu paste prepared by dispersing Cu and copper oxide and reduction-resistant glass frit added as the additives thereto into an organic medium. U.S. Pat. No. 3,647,532 (Friedman) discloses a paste prepared by dispersing Cu mixed with lead borosilicate glass frit containing cadmium oxide as the additive in an organic polymeric binder. This paste is fired at 820.degree.-850.degree. C. in a non-oxidizing atmosphere to form a conductor layer. Further, U.S. Pat. No. 4,070,518 (Hoffman) discloses a conductor paste for use, particularly, on a dielectric substrate. This paste comprises 85 to 97 wt% of Cu powder and 3 to 15 wt% of Cd- and Bi-free lead aluminoborate glass frit dispersed in an organic medium. U.S. Pat. No. 4,172,919 (Mitchell) discloses a conductor composition prepared by dispersing 86 to 97 wt% of Cu powder, 1 to 7 wt% of CuO and 1 to 7 wt% of glass frit containing at least 75% B.sub.2 O.sub.3 into 10 to 30 wt% of inert organic medium. U.S. Pat. No. 4,514,321 (Vincent P. Sivta) discloses a paste prepared by dispersing a Cu powder mixed with an inorganic binder and 0.2 to 5 wt% of non-copper metal into an organic solvent. The non-copper metal is selected from the group consisting of tungsten, molybdenum, rhenium and an alloy or a mixture thereof. Japanese Patent Publication No. 59-2398 discloses a thick film conductor composition comprising 65 to 90 wt% of inorganic ingredient composed of 86 to 99 wt% of Cu powder, 0.3 to 12 wt% of CuO powder, 0 to 12 wt% of PbO and 0 to 12 wt% of Bi.sub.2 O.sub.3 and 10 to 35 wt% of organic vehicle added thereto. These Cu conductor pastes as described above are used for microcircuits. However, in the case of using these Cu pastes, there are significant several problems regarding the method of manufacturing them.
At first, it is very difficult to control the atmosphere of the furnace to such an oxygen partial pressure that Cu does not suffer from oxidation and the organic ingredient contained in the Cu paste can completely be burnt out in the firing step. If the oxygen partial pressure is high, the Cu surface is oxidized to worsen the solderability and it also leads to reduction in the electric conductivity. While on the other hand, if the oxygen partial pressure is low, favorable adhesion cannot be obtained for the Cu metallization, and it is difficult to use the organic ingredient contained in the Cu paste. That is, the organic binder and the like used for the vehicle of the paste cannot completely be burnt (or eliminated). It is said that the organic binder does not decompose, particularly, at a temperature lower than the melting point of Cu (1083.degree. C.). It has been known that polyvinyl butyral often used is not easily burnt completely in a non-oxidizing atmosphere at a lower temperature than 1150.degree. C. Particularly, in the case of manufacturing a copper multilayer ceramic substrate, in which a glass-ceramic material is used as the insulation layer, it is necessary that the glass-ceramic material remains porous until all of the binder residues are removed upon the burn-out step. In the case of using the glass-ceramic and copper, the maximum temperature for removing the binder is much lower than about 800.degree. C.-875.degree. C. at which fine glass particles are fused. That is, if the glass particles are fused, the residual binder is captured in the glassy material. It has also been known that nitrogen or any other neutral or reducing atmosphere makes the burn-out, or the binder removing difficult at a temperature lower than the glass fusing temperature. As a result, black or blacky substrate not sintered completely are obtained. The color is generally due to carbon residues, which may result in the degradation of the insulation property (refer to Japanese Laid-Open Patent Application No. 55-128899).
Further, in the case of using metal Cu, since the metal Cu is oxidized into a copper oxide thereby causing volume expansion in the burn-out step (even if the Cu firing step is separated from the burn-out step), problems such as exfoliation of the conductor layer from the substrate are resulted. Furthermore, since the firing for the conductor layer and the insulation layer are repeated to form the multilayers, the respective layers are exposed to high temperature again and again to undergo damages. The increased number of steps increase the lead time thus resulting in the increased cost of the manufacturing installation.
While on the other hand, for attaining the complete binder removal, Japanese Laid-open Patent Application No. 53-129866 discloses a method of manufacturing a ceramic substrate capable of sufficiently removing the binder, which method comprises alternately printing a copper oxide paste and a dielectric paste comprising ceramics capable of sintering at a temperature lower than the melting point of Cu as the dielectric material to form a not-sintered ceramic substrate, provisionally firing the not-sintered material in the air atmosphere, and then firing same at 950.degree. C. in a reducing atmosphere. According to this method, the binder can be removed completely, control for the atmosphere during firing is facilitated and the co-firing is possible for the conductor layer and the dielectric layer. However, this method also involves a significant problem regarding the conductor paste. If the inorganic ingredient of the conductor paste comprises 100% CuO, since Cu has poor wettability with the ceramic substrate in the reducing atmosphere, favorable adhesion property cannot be obtained with respect to the substrate to result in the exfoliation of the conductor from the substrate. Further, when CuO is reduced into metal Cu, it causes a volume change (shrinkage), which results in disconnection between the upper and lower layer conductors.